Code Division Multiple Access- CDMA

CDMA – CODE DIVISION MULTIPLE ACCESS

VIVEK PATELCWID- 10404232

17/30/2015 EE 583 Code Division Multiple Access

Outline:

History And Introduction Multiple Access Method –

FDMA/TDMA/CDMA Comparison

Spread Spectrum CDMA Introduction

IS 95-CDMA General Specification How CDMA Works? Types Of CDMA

Architecture CDMA Architecture Spreading Codes Walsh Codes PN Sequences And

Orthogonal

CDMA Frame Structure

CDMA Capacity

Power control

Rake receiver

Handoffs

Detection

Third Generation 3G

Commercial use

Advantage And Disadvantage

Of CDMA

Conclusion

References 27/30/2015 EE 583 Code Division Multiple Access

1st.Generation(1980s)

Analog

NMT CT0TACS CT1AMPS

3rd. Generation

(2000s)2nd. Generation

(1990s)Digital

GSM DECTDCS1800 CT2PDC PHSIS-54IS-95IS-136UP-PCS

IMT-2000CDMA2000W-CDMA

Evolution of Cellular Systems :

7/30/2015 EE 583 Code Division Multiple Access 3

Time Time Time

Freq Freq Freq

PN Code

FDMA TDMA CDMA

Access Technology :


FDMA :

• In FDMA, the entire allocated cellular frequency spectrum is divided into a number of 30-kHz channels .

• The power transmitted by a cell is only large enough to communicate with mobile stations located near the edge of the cell’s coverage area.

• The radius of a cell might be one mile or less-referred to as a small cell.


6

FDMA

MS #1

MS #2

MS #n

BS

f1’

f2’

fn’

f1

f2

fn

…

……

Reverse channels

(Uplink)

Forward channels

(Downlink)


7

FDMA: Channel Structure

1 2 3 … NFrequency

Total Bandwidth W=NWc

Guard Band Wg

4

Sub Band Wc

Frequency

Protecting bandwidth

…

f1’ f2’ fn’

…

f1 f2 fn

Reverse channels Forward channels


TDMA :

• TDMA is a digital wireless air interface .

• It divides each carrier frequency into a number of time slots, each of which constitutes an independent telephone circuit.


C B A C B A C B A C B A

C

A

B

Time

f0

Frequency

9

MS #1

MS #2

MS #n

BS

…

…

Reverse channels

(Uplink)

Forward channels

(Downlink)

t

Frequency f ’#1 …#1 …

Frame

Slot

… #1 … #1

Frame

…t

Frequency f

Frame Frame

…t

#2 …#2 …

…t

#n … #n …

… #2 … #2…t

…#n …#n…t

TDMA:


10

TDMA: Channel Structure

… t

f #1 #2 #n #1 #2 #n…

(a). Forward channel

…#1 #2 #n

Frame FrameFrame

… t

f ’

#1 #2 #n #1 #2 #n…

(b). Reverse channel

…#1 #2 #n

Frame FrameFrame


11

TDMA: Frame Structure (Cont’d)

…

Time

Frequency f = f ’

#1 #2 #n #1 #2 #n…

Forward channel

Reverse channel

…#1 #2 #n

Forward channel

Frame Frame

#1 #2 #n…

Reverse channel

Channels in Simplex Mode


12

TDMA: Frame Structure (Cont’d)

…Time

Frequency#1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

Head DataGuard time


CDMA :

• A digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95.“

• One of the unique aspect of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number .

• Code division multiple access (CDMA) is a digital air interface standard, claiming eight to fifteen times the capacity of analog.


14

Code Division Multiple Access (CDMA)

MS #1

MS #2

MS #n

BS

C1’

C2’

Cn’

C1

C2

Cn

…

……

Reverse channels

(Uplink)

Forward channels

(Downlink)

Frequency f ’

Note: Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes, Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes

Frequency f


15

Comparisons of FDMA, TDMA, and CDMA Operation FDMA TDMA CDMA

Allocated Bandwidth

12.5 MHz 12.5 MHz 12.5 MHz

Frequency reuse 7 7 1

Required channel BW

0.03 MHz 0.03 MHz 1.25 MHz

No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10

Channels/cell 416/7=59 416/7=59 12.5/1.25=10

Control channels/cell

2 2 2

Calls/RF Channel 1 4* 40**

Voice channels/cell 57x1=57 57x4=228 8x40=320

Sectors/cell 3 3 3

Voice calls/sector 57/3=19 228/3=76 320

Capacity vs FDMA 1 4 16.87/30/2015 EE 583 Code Division Multiple Access

Spread Spectrum Principles :

7/30/2015 16EE 583 Code Division Multiple Access

• Does not attempt to allocate disjoint frequency or time slot resources • Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR

• Each user employs a noise-like wideband signal occupying the entire frequency allocation

• Each user contributes to the background noise affecting all users, but to the least extent possible.

• This restriction on interference limits capacity, but because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system


• Suppose each user use a wideband Gaussian noise carrier

• Suppose each user’s transmission is controlled so that all signals received at the BS are of equal power

• Let Ps be the power of each user, and the background noise be negligible.

• Then the total interference power, I, presented to each user’s demodulator is

I = (K-1) Ps (1) where K is the number of users



• Let’s say demodulator of each user operates at bit-energy-to-noise-density level of Eb/N0.

• So the noise density received by each user’s demodulator is N0 = I/W (2), where W Hz is the bandwidth of the wideband noise carriers.

• The received energy per bit is the received signal power divided by the data rate R (bits/s), i.e., Eb = Ps/R (3)



• Combining (1), (2) and (3) we getK – 1 = I/Ps = (W/R) / (Eb/N0) (4)

• If W >> R then the capacity of the system can be largei.e., transmission bandwidth should be much larger than the message bandwidth

• If Eb/N0 is small, then also the capacity can be large. (since Eb/N0 α SNR, this means SNR should be as small as possible)


Spread Spectrum (1) Spread Spectrum (1) ::

S(f)

ff0

Before spreading

Signal

S(f)

ff0

After spreading

Signal

S(f)

ff0

After despreading

signal

Interfering noise

f

S(f)

f0

Before despreading

Signal

Interfering noise


Spread Spectrum(2)Spread Spectrum(2) : :

Radio Channel

Channel Coding(SS)

Carrier Modulation

DS-PN

Source Coding

Transmit

Antenna

Radio Channel

ChannelCoding

(SS)

CarrierModulation

DS-PN

SourceCoding

Transmit

Channel Decoding

Carrier Demodulation

DS-PN

Source Decoding

Receive

Channel Decoding

CarrierDemodulation

DS-PN

SourceDecoding

Receive

Antenna

A B7/30/2015 EE 583 Code Division Multiple Access 21

Spreading Spectrum (3)Spreading Spectrum (3) “Principle of Using Multiple Codes” “Principle of Using Multiple Codes”


SpreadingSequence

A

SpreadingSequence

B

SpreadingSequence

C

SpreadingSequence

C

SpreadingSequence

B

SpreadingSequence

A

InputData

X

RecoveredData

X

X+A X+A+B X+A+B+C X+A+B X+A

Spread-Spectrum Chip StreamsORIGINATING SITE DESTINATION

Spread Spectrum Spread Spectrum ((44))


Advantages of Spread SpectrumAdvantages of Spread Spectrum::

1. Avoid interference arising from jamming signal or multi-path effects.

2. Covert operation : Difficult to detect

3. Achieve Privacy: Difficult to demodulate, Noise like signal.

4. Impossible to Eavesdrops on the signal expect using the same PN sequence


CDMA:

• Classification of CDMA Systems :

CDMA one

CDMA 2000

IS95 IS95B JSTD 008

Narrow Band

Wide Band

CDMA SYSTEMS


7/30/2015 26

CDMA Evolution PathsCDMA Evolution Paths

2Mbps

153.6kbps

CDMA1xRTT

CDMA 1xEV-DV

CDMAIS-95

CDMA1xEV-DO

CDMA3x 5x

2G 2.5G 3G

9.6kbps

EE 583 Code Division Multiple Access

CDMA-Its History & StatusCDMA-Its History & Status


CDMA-Its History & StatusCDMA-Its History & Status

1993, the first CDMA standard IS-95 was issued;In 1995, CDMA technology was put into commercialization in

Hong Kong and America on large scale;In April, 2001, China Unicom began to construct CDMA

networks—the largest in the world (about 70Million line now);At present, CDMA commercial networks are established in

about 40 countries or area, almost 20% of all users in the world.


CDMA Subscriber Growth History:CDMA Subscriber Growth History:Sept.1997 through Sept.2003Sept.1997 through Sept.2003


IS-95 CDMA:

• IS-95 (cdmaone) 2G digital cellular standard

•Motivation–Intended as a new system (greenfield) or replacement for AMPS (not an upgrade)

–Increase system capacity–Add new features/services


IS-95 CDMA:

•History:–1990 Qualcomm proposed a code division multiple access (CDMA) digital cellular system claimed to increase capacity by factor 20 or more

–Started debate about how CDMA should be implemented and the advantages vs. TDMA (religious tones to debate)

–1992 TIA started study of spread spectrum cellular

•Several alternative CDMA proposals floated – large

debate in the CTIA–came down to Interdigital vs. Qualcomm–Qualcomm proposal won


IS-95 CDMA:

• 1993 TIA IS-95 code division multiple access

• (CDMA) standards completed–1995 IS-95A enhanced revision

–ANSI J-STD-008 (IS-95b) is standard up banded to

1900 MHz PCS band

–1996 Commercial deployment in US (Sprint PCS)–Most popular system in U.S. and Korea–1997 IS-95 name changed to cdmaone

• IS-95 evolves to cdma 2000 in 2.5 and 3G


IS-95 System Features:•Digital Voice

–QCELP fixed rate 14.4Kbps coder–variable rate QCELP coder: 9.6, 4.8, 2.4, 1.2 Kbps

•Use of voice activation to reduce interference•As data rate reduces, the transmitter can reduce the power to achieve the same error rates

•Dual Mode (AMPS/CDMA), Dual Band (900, 1900 MHz bands)

•Low power handsets (sleep mode supported)•Soft Handoff possible•Digital Data services (text, fax, circuit switched data)•Advanced Telephony Features (call waiting, voice mail, etc.)


IS-95 System Features:

•Security: CDMA signal + CAVE encryption•Air Interface Standard Only•Code Division Multiple Access/FDMA/FDD•Traffic Channel

–Pair of 1.25 MHz radio channels (up/downlink)

–Several users share a radio channel separated by a

code not a timeslot or frequency!–Receiver performs a time correlation operation to

detect only desired code word–All other code words appear as noise due to decorrelation

–Receiver needs to know only codeword and frequency used by transmitter7/30/2015 EE 583 Code Division Multiple Access 34

IS-95 System Features:

–Adjust power often to prevent near –far problem

•Universal frequency reuse (frequency reuse cluster

size K =1)–Simple planning–large capacity increase


IS-95 Radio Aspects:


Code Division Multiple Access – CDMA:

• Multiple users occupying the same band by having different codes is known as CDMA - Code Division Multiple Access system

Let W - spread bandwidth in Hz

R = 1/Tb = Date Rate

S - received power of the desired signal in WJ - received power for undesired signals like multiple access users, multipath, jammers etc in W

Eb - received energy per bit for the desired signal in W

N0 - equivalent noise spectral density in W/Hz7/30/2015 37EE 583 Code Division Multiple Access


7/30/2015 38

• What is the tolerable interference over desired signal power?

min0max NE

RW

S

J

b


00

0

NE

RW

NE

WT

TE

WN

S

J

bb

b

bb

)()((db)margin Jammingmin0max

dbN

Edb

R

W

S

J b


• In conventional systems W/R 1 which means, for satisfactory operation J/S < 1

• Example Let R = 9600; W = 1.2288 MHz(Eb/N0)min = 6 dB (values taken from IS-

95)Jamming margin (JM) = 10log10(1.2288*106/9.6*103) - 6

= 15.1 dB 32• This antijam margin or JM arises from Processing Gain (PG) = W/R = 128

• If (Eb/N0)min is further decreased or PG is increased, JM can be further increased7/30/2015 39EE 583 Code Division Multiple Access


• JM can be used to accommodate multiple users in the same band

• If (Eb/N0)min and PG is fixed, number of users is maximized if perfect power control is employed.

• Capacity of a CDMA system is proportional to PG.



• Multiple Access in CDMA:oEach user is assigned a unique PN code.oEach user transmits its information by spreading with unique code.

oDirect Sequence spread spectrum is used.oUsers are separated by code not by time slot and frequency slot.



• Concept of CDMAoUsers share same bandwidthoUser axis shows cumulative signal strength of all users

Code 1

Code2

Code 3

Code 4

Freq

User


-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 50000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1noise

massegeAfter SpredingChip Duration is 0.4

ms.Bit Duration is 6.4 ms.Processing Gain is 16

How CDMA works?:

• Spectrum of the CDMA:


Types of CDMA: 1. Frequency hopping

• Fixed sequence of frequency values & time is divided into slots .

• In the first time slot, a given user transmit to the base station using the first frequency in its frequency hopping sequence.

• In the next time interval, it transmits using the second frequency value in its frequency hopping sequence, and so on.

• This way, the transmit frequency keeps changing in time.


Types of CDMA:

2. Direct sequence

• Each user transmits its message to the base station using the same frequency, at the same time.

• Here signals from different users interfere with each other.

• But the user distinguishes its message by using a special, unique code. This code serves as a special language that only the transmitter and receiver understand.


ME

SIM

BTS

BTS

BSC

BSC

MSC

VLR

EIR

HLR

AuC

AUm

Abis

PSTNISDN

Mobile station

Base Station Subsystem

NetworkSubsystem

MS BSS NS

Architecture Of CDMA:



• Mobile Station (MS): Two Blocks

oMobile Equipment (ME)oSubscribers Identity Module (SIM)

• Function of Mobile Station:

1. Personal Mobility 2. IMEI (International Mobile Equipment Identity) 3. IMSI (International Mobile Subscriber Identity)


BSC


•Base Station Subsystem (BSS)

oBase Transceiver Station (BTS)oBase Station Controller (BSC)



• BTS : Base Transceiver station

• 1. It defines the cell .

• 2. It handles the radio link protocol with the mobile station

BSC: Base station Controller

• 1. It manages radio resources for one or more BTS.

• 2. Allocation and Deallocation of channels.

• 3. Transmitter power control.

• 4. Handoff control


BSC MSC

VLR

EIR

HLR

AuC

PSTN

ISDN


• Network SubsystemoMSC: Mobile Switching CenteroHLR: Home Location RegisteroVLR: Visitor Location registeroAuC: Authentication CenteroEIR: Equipment Identity Register



• Mobile Switching center:(MSC)

1 Call set up/supervision/release 2 Call routing 3 Billing information collection 4 Mobility management 5 Paging, Echo cancellation connection to BSC, other MSC and other local

exchange networks 6 Access to HLR and VLR



• Home Location Register (HLR) 1 One HLR per CDMA operator 2 Contains permanent M database of all the subscribers in the network

3 Contains MSRN(mobile station routing no.) 4 It is referred for every incoming call



• Visitor Location Register(VLR) 1. Temporary visitors database 2. One VLR per MSC

• Authentication Center(AuC) 1. Provides security 2. Authentication and encryption

• Equipment Identity Register: 1. Contains IMEI


Spreading Codes:

• A noise-like and random signal has to be generated at the transmitter.

• The same signal must be generated at the receiver in synchronization.

• We limit the complexity by specifying only one bit per sample, i.E., A binary sequence.


Desirable Randomness Properties Of SC:

• Relative frequencies of “0” and “1” should be ½ (balance property)

• Run lengths of zeros and ones should be (run property):• Half of all run lengths should be unity• One - quarter should be of length two• One - eighth should be of length three• A fraction 1/2n of all run lengths should be of length n for all finite n


Desirable Randomness Properties Of SC:

• If the random sequence is shifted by any nonzero

number of elements, the resulting sequence

should have an equal number of agreements and

disagreements with the original sequence

(Autocorrelation property)


Walsh CodesWalsh Codes::

• 64 sequences, each 64 chips long• A chip is a binary digit (0 or 1)

• Each Walsh code is orthogonal to all other Walsh codes• This means that it is possible to recognize and therefore extract a particular Walsh code from a mixture of other Walsh codes which are “filtered out” in the process

• Two same-length binary strings are orthogonal if the result of xoring them has the same number of 0s as 1s


PN Sequences:

• A deterministically generated sequence that nearly satisfies these properties is referred to as a Pseudorandom Sequence (PN)

• Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR)

• If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence


PN Sequences:

• However, if the feedback connections satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence.

• Thus if r=15, P=32767.

• MLSR satisfies the randomness properties stated before


Randomness Properties of PN Sequences:

• Balance property - Of the 2r - 1 terms, 2r-1 are one and 2r-1–1 are zero. Thus the unbalance is 1/P. For r=50; 1/P10-15

• Run length property - Relative frequency of run length n (zero or ones) is 1/ 2n for n r-1 and 1/(2r - 1) for n = r

• One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r

• Autocorrelation property - The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p


PN Sequences Specified in IS-95:

• A “long” PN sequence (r =42) is used to scramble the user data with a different code shift for each user

• The 42-degree characteristic polynomial is given by:• x42+x41+x40+x39+x37+x36+x35+x32+x26+x25+x24+x23+x21+x20

+x17+x16+x15+x11+x9+x7+1

• The period of the long code is 242 - 1 4.4*102 chips and lasts over 41 days


PN Sequences Specified in IS-95:

• A short PN sequence (r = 15) is specific to a base station and its period is (215−1)Tc = 27ms.

• Two “short” PN sequences (r=15) are used to spread the quadrature components of the forward and reverse link waveforms


CDMA Related Terms:

• CDMA Channel or CDMA Carrier or CDMA Frequency:Duplex channel made of two 1.25 MHz-wide bands of electromagnetic spectrum, one for Base Station to Mobile Station communication (called the FORWARD LINK or the DOWNLINK) and another for Mobile Station to Base Station communication (called the REVERSE LINK or the UPLINK).

• In 800 Cellular these two simplex 1.25 MHz bands are 45 MHz apart,

• In 1900 MHz PCS they are 80 MHz apart


CDMA Related Terms:

• CDMA Forward & Reverse Channel1.25 MHz Forward / Reverse LinkCDMA Code ChannelEach individual stream of 0’s and 1’s contained in either the CDMA Forward Channel or in the CDMA Reverse Channel

• Code Channels are characterized (made unique) by mathematical codesCode channels in the forward link: Pilot, Sync, Paging and Forward Traffic channelsCode channels in the reverse link: Access and Reverse Traffic channels


cdma2000 Uplink Frame Structure:

• Radio Configuration 3

7/30/2015 65

CRCencodertail bits

Convolutionalor Turbo Coder

symbolrepetition

symbolpuncture

blockinterleaver

modulation symbol

C

channel bits

Bits/Frame1640 80 172350744151230486120

CRC bits668121616161616

tail bits888888888

Data Ratekbps1.52.74.89.619.238.476.8153.6307.2

CodeRate1/41/41/41/41/41/41/41/41/2

Repeats1684211111

Delete1 of 51 of 9nonenone none none none none none

Symbols153615361536153615363072 6144 12288 12288


cdma2000 Uplink Modulator:

7/30/2015 66

C

C

C

A

B

d

d

d

c

)cos( tc

)sin( tcIlong,c

Qlong,c

+

+

+

+

+

_

sum

sum1 Dw

Cw

2 Sw

1 Sw

PrimaryTraffic

SecondaryTraffic 1

SecondaryTraffic 2

Control

Pilot

pulseshape

pulseshape


cdma2000 Downlink Frame Structure:

7/30/2015 67

Bits/Frame2155 125 267552112822804584919220712

CRC bits681012161616161616

tail bits8888888888

Data Ratekbps1.83.67.214.428.857.6115.2230.4460.81036.8

CodeRate1/21/21/21/21/21/21/21/21/21/2

Repeats8421111111

Deletenonenonenonenone none none none nonenone2 of 18

Symbols57657657657611522304 4608 92161843236864

Radio Configuration 9

CRCencodertail bits

Convolutionalor Turbo Coder

symbolrepetition

symbolpuncture

blockinterleaver

modulation symbol

channel bits

W


Benefits of the cdma2000 1x StandardsBenefits of the cdma2000 1x Standards::

• Increased mobile standby battery life (via Quick Paging Channel)

• Total backward compatibility to reuse switch and call processing features

• 2-3 dB better coverage• High speed 153.6 kbps packet data capabilities• cdma2000 1x = 1.25 MHz Radio Transmission Technology


cdma2000 1X DL Modulation Processing:


+

longcode

0 11 -1

channelgain

fwd pwrctrl gain po

wer

con

trol

sym

bol p

unc

ture

DE

MU

X

decimate pwr ctrlbit pos

I/Q scrambling bit extract

powercontrol

bits

puncturetiming800 Hz

W

YQ

YI

modulationsymbol

rate

cdma2000 1X Downlink Modulation:

7/30/2015 70

I channelpilot PN

Q channelpilot PN

Walsh code

QOF code

IY

QY

pulseshape

pulseshape

+

+

+

-

)cos( tc

)sin( tc

otherchannels

otherchannels

pha

se

rota

te f

or Q

OF

QX

IX


CDMA Capacity:

•CDMA Main Advantages–resistant to narrow band interference

–resistant to multipath fading and ISI

–no hard limit on number of users (soft capacity)


.1

.01

.001

.0001

10 20 30 40 50 60

users

BER of CDMA system with 128 cps.

CDMA Capacity:

–As number of users on a frequency increase the interference level increases and BER increases for all users

–With proper limits all frequencies can be used in every cell


CDMA Capacity:

•CDMA is a interference limited system–Must limit number of users on a frequency to limit interference within a cell and between cells using same frequency (All CDMA carriers can be assigned to each

sector in each cell!)

–Total Interference It = Ioc + Io + No

•Ioc = other cell interference, Io = own cell interference, No = Noise

•uplink not downlink in CDMA systems considered the constraining factor


CDMA Capacity:

•Remember in direct sequence spread spectrum Processing Gain = bandwidth of the spread signal to the bandwidth of the data signal = W/R–In IS-95 W/R = 10 log (1.23.MHz/9.6 KHz) = 21.1 dB for rate set 1, for rate set two (14.4 kbps) => 19.3 db

•Number of traffic channels per carrier and cell function of processing gain, interference, speech coder tolerance for errors, error control coding, etc.


CDMA Capacity:

• The effect of more users in a cell on frequency is to degrade the channel for everyone – can be thought off as decreasing the usable cell size


Power Control in CDMA:

• CDMA goal is to maximize the number of simultaneous users

• Capacity is maximized by maintaining the signal to interference ratio at the minimum acceptable

• Power transmitted by mobile station must be therefore controlled

• Transmit power enough to achieve target BER: no less no more


Power Control in CDMA:

• Controlling transmitted power affects the CIR


77

Pr

Pt =

1

4df

c

Pt = Transmitted powerPr = Received power in free spaced = Distance between receiver and transmitterf = Frequency of transmissionc = Speed of lighta = Attenuation constant (2 to 4)

Power Control: Open Loop vs. Closed Loop

•Open loop:–Base station transmits at a known power level (a beacon) which mobile measures to estimate the path loss

–Assumes path loss in both directions is the same–Not very accurate as uplink and downlink are separated in frequency

–Useful for coarse initial estimates at mobile used in Access channel for signaling


Power Control: Open Loop vs. Closed Loop:

•Closed loop:–Signal-to-Interference Ratio (SIR) measured at the receiver and compared to a target value for SIR

–Receiver sends a power control command to transmitter to reduce or increase the power level - requires a bi-directional link

–Used in TCH – power control sub channel operates at 800 bps by puncturing downlink data with periodic bits – each power command adjusted MS power in 1 dB increments


Closed Loop Power Control: Inner Loop vs. Outer Loop:

• Inner Loop (or fast power control)–Measures received SIR, controls transmit power–Commands sent several times per frame (hence fast power control)


Closed Loop Power Control: Inner Loop vs. Outer Loop:

•Outer Loop (or slow power control)–Measures packet error rate–Changes target SIR for inner loop–Directly modify transmit power based on FER–Commands sent once per frame (hence slow power control)


Two factors important for power control:

• Propagation loss • due to propagation loss, power variations up to 80 dB

• a high dynamic range of power control required

• Channel Fading • average rate of fade is one fade per second per mile hour of mobile speed

• power attenuated by more than 30 dB• power control must track the fade


Power Control in IS-95A:

• At 900 MHz and 120 km/hr mobile speed Doppler shift =100Hz

• In IS 95-A closed loop power control is operated at 800 Hz update rate

• Power control bits are inserted (‘punctured’) into the interleaved and encoded traffic data stream

• Power control step size is +/- 1 dB• Power control bit errors do not affect performance much


Rake Receiver:

• Mobile station receives multiple attenuated and delayed replicas of the original signal (multipath diversity channels).

• Two multipath signals are resolvable only if their relative delay exceeds the chip period Tc

• Amplitudes and phases of multipath components are found by correlating the received waveform with multiple delayed versions of the signal (delay = nTc).

• Searcher performs the above task for up to 3 different multipath signals.

• 3 parallel demodulators (RAKE fingers) isolate the multipath components and the RAKE receiver combines them.


d1 d2

t t t

d3

transmission

receivingRacer combination

noise


Rake Receiver:

Correlator 1

Correlator 2

Correlator 3

Search Correlator

Com

bine

r

To De-Interleaver, Viterbi Decoder

Multipath Delay Components

( 150 s > t > 1s)

Rake receiver can isolate multipath spaced > 1 chip length.


Rake Receiver:

• Handset uses combined outputs of the three traffic correlators “rake fingers”

• Each finger can independently recover a particular PN offset and Walsh code

• Fingers can targeted on delayed multipath reflections, or even on different BTSs

• Searcher continuously checks pilots


Rake Receiver:

7/30/2015 88

88

Handoff in CDMA System:

• In GSM hard handoff occurs at the cell boundary• Soft Handoff

• Mobile commences Communication with a new BS without interrupting communication with old BS

• same frequency assignment between old and new BS• provides different site selection diversity

• Softer Handoff• Handoff between sectors in a cell

• CDMA to CDMA hard handoff • Mobile transmits between two base stations with different frequency assignment


Handoff is of 3 types1. Intra BSC2 Inter BSC3. Inter MSC

Handoff in CDMA System:

BSCBSC BSC

MSC MSC

GMSC


Soft Handoff- A unique feature of CDMA Mobile:

Advantages• Contact with new base station is made before the call is switched

• Diversity combining is used between multiple cell sites• Diversity combining is the process of combining information from multiple transmitted packets to increase the effective SNR of received packets

• additional resistance to fading • If the new cell is loaded to capacity, handoff can still be performed for a small increase in BER

• Neither the mobile nor the base station is required to change frequency


Detection in the CDMA system:

• Multi-Users Detection (MUD)• The capacity and the performance of the CDMA system increase when MUD is implemented.

• The basic principle of MUD is the elimination of the negative effect of each user on the other.

• MUD is also known as joint detection and interference cancellation .

• Multi-user detection considers all users as signals for each other.



• Multi-Users Detection (MUD)• The ultimate technique for the MUD is the optimum receiver.

• Optimum Receiver:• Very complex. So, it is unpractical solution to reduce the multiple access interference. Sup-optimum detector will be implemented.



• Multi-Users Detection (MUD)• Sup-optimum solution:

• Serial Interference cancellation (SIC):• SIC is preferred in the absence of power control.

• Serial canceling the interference generated from the other users.

• Cancel the strongest signal first (most negative effect )



• Multi-Users Detection (MUD)• Parallel Interference Cancellation (PIC):

• PIC would be preferred when the amount of interference from each user is similar.

• In the initial detection stage, all active users are detected in parallel by a SUD technique.

• Then, the obtained interference is subtracted from the received signal and data detection is performed again with reduced MAI



Disadvantages:1. Multiple Access Interference:

• Require multi-user detection (MUD) algorithms to solve.

2. Near-far problem.• Where stronger (near to the Base Station) user masks the weaker user (far from the Base Station)

3. Requires wideband channel.


What is Third Generation? :

• Flexible support of multiple services• Voice• Messaging – email, fax, etc.• Medium-rate multimedia – Internet access, educational

• High-rate multimedia – file transfer, video• High-rate interactive multimedia – video teleconferencing, telemedicine, etc.

• Mobility: quasi-stationary to high-speed platforms• Global roaming: ubiquitous, seamless coverage• Evolution from second generation systems


W-CDMA Versus cdma2000 :

7/30/2015 97

97


cdma2000 vs WCDMA:

• Chip rate• Coherent Pilot Channels• Transmit Diversity• Underlying Network• Single Carrier versus Multicarrier Spreading• Cell Site Synchronization


Commercial usage of CDMA:

• CDMA was introduced in the 2nd Generation (early of the 1990s).• E.g. IS-95 standard, also known as cdma One which support up to 64 users that are orthogonally coded and at the same time transmitted over 1.25 MHz channel

• Used for Cellular Communication System • (824-894 MHz in US Cellular)

• 3rd Generation: cdma2000• Allow high rate of packet transmitting in addition of voice transmitting.


CDMA AdvantagesCDMA Advantages::

• Spread Spectrum• Soft & Softer Handoff• Rake Receiver• Variable Rate Vocoder• High quality voice• Power Control• Coverage• Simple Network Planning• Green Handset• Smooth migration to 3G and the operator’s benefit is protected at the most


DISADVANTAGES OF CDMA :

• Near far problem.

• CDMA is relatively low, and the network is not as mature as GSM.

• CDMA can not offer international roaming, a large GSM advantage.

Conclusion:

• The CDMA will allow many signals to be transmitted at the same channel at the same time. This done by giving each user a Pseudo-Noise code which is a binary sequence. This code should have low cross-correlation between each other.

• Multiple access interference has bad effect on the CDMA system so the multiple user detection is used to reduce the MAI.• CDMA cellular system is deemed superior to the FDMA and TDMA

• cellular systems for the time being. 7/30/2015 102EE 583 Code Division Multiple Access

Conclusion:

• Therefore, CDMA technique becomes more important in radio

• communication systems.

• CDMA is based on the spread spectrum technique which has been used at military field.


References:

• http://www.tubex.com/category/cdma/gsm• http://www.bsnl.co.in:cdma/architecture.html• http://www.cellular.co.za/technologies/cdma/cdma_w_paper.htm

• http://www.arcx.com/sites/CDMAvsTDMA.htm• Lee JS and Miller LM, CDMA System Engineering Handbook, Arttech Publishing House, 1998.

• Viterbi A, CDMA-Spread Spectrum Communication, Addison Wesley 1995.

• R. L. Peterson, “Third Generation Personal Communications: Physical Layer Status,” Presentation at Clemson University, Feb. 1, 2001


http://www.tubex.com/category/cdma/gsm

http://www.cellular.co.za/technologies/cdma/cdma_w_paper.htm

http://www.cellular.co.za/technologies/cdma/cdma_w_paper.htm

http://www.arcx.com/sites/CDMAvsTDMA.htm

References:

• Manjit Singh and Manoneet Singh, “3G Wireless with Respect to IMT-2000 and Beyond,” Telecom 99

• Harri Holma and Antti Toskala, WCDMA for UMTS: Radio Access for Third Generation Mobile Communications, New York: Wiley, 2000

• “CDMA Evolution from IS-95, IS-2000, to 1XTREME,” Technology Transfer Training Class, Motorola, Inc., July 2000


Code Division Multiple Access- CDMA

Engineering

Transcript of Code Division Multiple Access- CDMA